1. Field of the Invention
The present invention relates to a mounting structure for an assembly of an engine and a transmission in a vehicle, and in particular to such a mounting structure which is well adapted for mounting an internal combustion engine and a transmission laterally to the vehicle body and for limiting the occurrence of undesirable vibration.
2. Discussion of the Background
In a vehicle such as an automobile, the coupled combination of the engine and the transmission is mounted in the engine room of the vehicle by being suspended therein upon mounting devices which can incorporate both spring means and damping means, such as for example rubber engine mounting blocks, in order to better control engine and body vibration. However, because at various different times the engine is prone to various different vibrations of various different frequencies and amplitudes, prior art mounting systems have not always been found satisfactory, because such mounting systems of a particular spring constant and a particular damping coefficient which were able to control to some extent some sort of vibration of the engine were not able to control other sorts of vibration of the engine. Further, the difficulty has arisen that vibration of the engine-transmission assembly in one vibrational mode has tended to excite vibration in another vibrational mode; for example, rolling vibration of the assembly has sometimes tended to excite vibration in the vertical direction. This is termed coupling of vibrational modes.
In detail, the engine-transmission assembly in a vehicle is liable to undergo five main different types of vibration, hereinafter termed idling vibration, stumbling, engine shaking, engine wind up vibration, and booming.
The idling vibration of an engine-transmission assembly is a phenomenon that the engine-transmission assembly rolls vibrationally about the one axis of its principal axes of inertia which typically is nearly parallel to and nearly coincident with the rotational axis of the power output member of the engine-transmission assembly, during idling operation of the engine (which is one of the minor axes of inertia), and this occurs when the resonant frequency of rotation of the engine-transmission assembly around this principal axis of inertia coincides with or is an exact multiple or submultiple of the rotational speed of the engine. This idling vibration is a particular problem in a vehicle in which the engine-transmission assembly is transversely mounted, because the vibration is particularly effectively transmitted to the vehicle body in such a case. In order to suppress this idling vibration of the engine-transmission assembly, the spring constant and the damping coefficient of the engine supports that are particularly concerned with this vibration are both required to be low, so as to lower the resonant frequency of rotation of the engine-transmission assembly around this principal axis of inertia, and so as not to present a high apparent spring constant as explained below. The amplitude of this idling vibration tends to be quite low, while the frequency is medium to quite high.
Stumbling is a phenomenon wherein the vehicle is vibrationally jerked in the fore and aft directions during rapid acceleration or deceleration of the vehicle, and this is due to twisting or rolling vibration of the engine-transmission assembly caused by rapid changes of torque supported by the engine-transmission assembly housing relative to the output shaft of the engine. In order to suppress this stumbling, the spring constant and the damping coefficient of the relevant parts of the engine-transmission assembly mounting system are both required to be high. The amplitude of this stumbling tends to be quite high, while the frequency is quite low.
Engine wind up vibration is a phenomenon wherein the engine-transmission assembly vibrates up and down relative to the vehicle body by being coupled to the rolling vibration of the engine-transmission assembly, when the vehicle is suddenly accelerated or decelerated; and this occurs when the natural frequency of the relevant parts of the engine-transmission assembly mounting system in the vertical direction resonates with the frequency of such rolling vibration of the engine-transmission assembly. In order to suppress this engine wind-up vibration, it is required to uncouple the vertical vibration of the engine-transmission assembly from the rolling vibration thereof.
Engine shaking is a phenomenon wherein the engine-transmission assembly vibrates up and down relative to the vehicle body during operation of the vehicle upon an irregular road surface, due to the bumping of the vehicle body up and down upon the irregular road surface, said bumping of the vehicle body being transmitted to the engine-transmission assembly by way of the mounting system therefor, and this occurs when the natural frequency of the relevant parts of the engine-transmission assembly mounting system in the vertical direction resonates with the up and down vibrational bouncing movement of the vehicle body. In order to suppress this engine shaking, the spring constant and the damping coefficient of the relevant parts of the engine-transmission assembly mounting system are both required to be high. The amplitude of this engine shaking tends to be quite high, while the frequency is quite low.
Finally, the booming is a phenomenon wherein a booming noise is generated, typically during high speed relatively steady operation of the vehicle, in the passenger compartment of the vehicle due to vibrations of relatively high frequency of the engine-transmission assembly and the drive train of the vehicle which are consonant with the natural frequency of the vehicle body, particularly the compartment structure of the vehicle, thus causing a booming noise in the passenger compartment. In order to suppress this booming noise, the spring constant and the damping coefficient of the relevant parts of the engine-transmission assembly mounting system are both required to be low. The frequency of this booming is quite high, being about 100 Hz or so. An additional characteristic of the booming vibration of the engine-transmission assembly and the vehicle body is that the amplitude of vibration is much smaller than those of the other above mentioned types of vibration of the engine-transmission assembly, being in the neighborhood of 15 microns or so in half wave amplitude.
Now, it should be understood that the spring constant of a combination device for coupling two members together, which is made up of a combination of a spring type or resilient mounting device and a damping device, in principle is of course solely due to the springing device, and in principle is quite independent of the damping effect or damping coefficient of the damping device. However, at any particular frequency of applied vibration, there is a so called apparent or effective spring constant, which is a combination of the actual spring constant and a contributory apparent spring constant element due to the operation of the damping device which increases along with increased damping coefficient of the damping device. Thus, when the applied force is a periodic vibration, if the damping device has a lower damping coefficient, it will appear that the spring effect of the combination as a whole is softer; and, contrariwise, if the damping device has a higher damping coefficient, it will appear that the spring effect of the combination as a whole is harder. Further, the higher the increase in the effective spring constant due to the action of the damping device, the higher is the frequency of vibration applied to the combination device; in other words, the higher these values are, the greater is the rate of change of position between the two members which are being coupled together by the combination device. The details of the exact mathematics involved are beyond the scope of the present specification, for which the empiricial discussion above is sufficient.
It is of course desirable that the mounting of the engine-transmission assembly in the vehicle should be accomplished by as simple a means as possible. Also it is important that the arrangement and type of the mounting means should be appropriate for suppressing these various types of vibration explained above. Since the various forms of vibration each have a particular directivity, and since a particular spring constant and a particular damping coefficient are appropriate for controlling each of these forms of vibration, a proper choice of the number and positioning of the mounting devices for the engine-transmission assembly is very important for providing proper vibration control.